1. Field of the Invention
The present invention relates to sensing blood velocities in tissues such as, but not limited to, skin, skin flaps, transplants, breasts, retinas, and internal organs and tissue.
2. Discussion of the Related Art
The evaluation of hemodynamics is an important diagnostic subject and has been one of the most difficult challenges in medicine. In studies of skin it is important to assess blood velocities over the area of interest to determine blood perfusion and predict tissue viability. For surgical procedures involving skin flaps, a reliable method of quantitatively monitoring tissue blood velocity can provide predictive value in assessing tissue conditions during partial detachments, and before, during, and after reattachment to avoid tissue necrosis. The same is true in the transplant of tissues and organs, and before, during, and following surgery.
It is important to diagnose tissue damage due to complications of diabetes, address practicality and viability of tissue repairs and vascular densities, and detect angiogenesis in large sites being studied for possible carcinomas, for example, breast cancer. It is also important to assess blood perfusion and blood velocities in real-time and to be able to provide the information to attending medical personnel in manners that are readily perceivable and understandable. Accordingly, there is a need for assessing surgical procedures regarding reconstructive surgery involving flaps, the treatment of vascular diseases, the condition of diabetic complications, the progression of tumors, and monitoring the status of surgically implanted flaps.
Free tissue transfer is a routine surgical procedure. Complications generally occur within 48 hours of the initial surgery. Tissue necrosis sets in if poor tissue perfusion is not corrected within 12 hours of surgery. The need for early detection of vascular insufficiency in free flaps is important since the success of corrective surgery strongly depends on the time elapsed since the onset of vascular insufficiency. Between 12% and 17% of flap surgery cases require re-exploration due to post-operative vascular complications that threaten flap viability. Flap salvage rates can be as high as 50%, depending on the procedure and the elapsed time since the onset of vascular occlusion.
Flap viability can be assessed by clinical observations of flap color, tissue turgor, capillary refill, and bleeding after a pinprick. Tissue viability monitoring techniques include laser Doppler velocimetry (LDV), differential thermometry, transcutaneous oxygen measurement, plethysmography, and Doppler ultrasound. Clinical visual observation remains the standard for assessing tissue viability. Early detection of decreased blood supply to the flap can prevent wide-scale tissue necrosis and eliminate the need for additional surgical procedures.
Measurement of retinal blood velocities is an important application of the invention. The retina provides direct optical access to both the central nervous system (CNS) and afferent and efferent CNS vasculature. This unique feature has provided generations of ophthalmologists with the ability to evaluate multi-system diseases without invasive diagnostic testing using direct opthalmoscopy, indirect opthalmoscopy, and slit lamp biomicroscope examination utilizing 90 or 78 diopter lenses, and the Hruby lens. These methods, however, cannot directly and reproducibly quantify retinal blood velocity, nor do they detect preclinical alterations predictive of eventual significant morbidity. This is particularly pertinent to the insidious onset of glaucoma and macular degeneration. The trend toward preventive medicine prescribes a more sensitive technique to reliably quantify subtle changes in retinal hemodynamics.
Both incoherent and coherent optical techniques have been used to assess microcirculation. The incoherent approach includes the fluorescein dye dilution method and the blue field entoptic method for retinal blood velocity measurement, and plethysmography. The coherent approach is represented by the laser Doppler method and the dynamic laser speckle method. The former employs a focused laser beam to measure the frequency shifts of radiation scattered by a scatterer. It requires a scanning mechanism for imaging applications. Its application to turbid media requires a consideration of multiple scattering effects. The dynamic laser speckle technique has been used for both point measurements and imaging applications in cases where multiple scattering is not prominent, e.g., monitoring blood and lymph flow in microvessels and in visualizing retinal microcirculation. Taking advantage of the advanced digital photography, the Laser Speckle Contrast Analysis (LSCA) technique extends the conventional laser speckle method to a nonscanning, full-field technique.
Needs exist for improved real-time measurement of blood perfusion and velocities. The needs are especially important in skin, skin flaps, surgical sites, transplants, breasts, and retinas. In the related art (U.S. Pat. No. 7,113,817), the system is started 60, aimed and focused 62, as shown in FIG. 1. The camera shutter exposure time, detector gain and aperture are separately set 64. A decision is made 66 to see if the target tissue is in the view finder. If the answer is no 68, a return to the aim and focus step 62 is required. If the answer is yes 70, the trigger shutter 72 is tripped, and the PC interrogates the detector to obtain a visual image 74. A decision is made 76 to see if the visual image contains the targeted tissue. If the answer is no 77, a return to step 62 is required. If the answer is yes 78, the system decides whether to obtain a laser speckle image 80. If a laser speckle image is not desired 82, the system is stopped 84.
If a laser speckle image is to be obtained 86, the laser is turned on 88, and the laser is aimed 90 at the target tissue. A laser filter 92 is inserted. The exposure time, detector gain and aperture are set 94, and the shutter is triggered 96. The detector is interrogated 98 to obtain a laser speckle image, and it is determined 100 if there are any saturated pixels. If saturated pixels exist 102, the system returns to adjust the exposure time, detector gain and/or aperture 94. If there are no saturated pixels 104, speckle contrast is computed 106 from the data obtained from the detector. The system uses multiple scattering corrections to obtain characteristic velocity 108. It maps the characteristic velocity onto the image of the tissue 112, displays the velocity mapping 110, and archives the data 114. An inquiry is made whether it is desired to obtain another image 116 of the same view from the same sample. If the answer is yes 118, the system returns to step 94 and sets the exposure time, detector gain and aperture for another image. If it is not desired to obtain another image 120, a decision is made 122 whether to obtain a different view. If the answer is yes 124, the system returns to the aim and focus step 62. If the answer is no 126, the system stops 84.
Although the related art system in U.S. Pat. No. 7,113,817 can use multiple scattering corrections to obtain characteristic velocity 108, the system in the related art does not use different images obtained at different exposure times of the same scene to create a combination image by dynamically correcting the geometric dependent parameter (the optical coherence parameter β) and/or the number of times a photon collides with red blood cells (m). This invention is an improvement upon the LSCA/MS technique described in U.S. Pat. No. 7,113,817, which is hereby incorporated by reference in its entirety. The present invention improves the accuracy of the blood velocity measurement using algorithms that compute blood velocity by selectively combining multiple images, thereby dynamically correcting the geometric dependent parameter (the optical coherence parameter β) and/or the number of times a photon collides with red blood cells (m) to obtain a more stable, reproducible, and accurate measurement of blood velocity.